A membrane receptor assay will be developed for measurement of insulin in biological fluids. The assay will depend on biologic activity of insulin and permit a determination of whether insulin circulating in the blood is biologically active in patients with certain kinds of diabetes and obesity in whom measurements by other methods show levels of insulin which do not correspond with the expected biologic activity of insulin. A new method of labeling insulin will be used, acetylation. This method has advantage over currently used methods for labeling insulin such as greater probability of preservation of biological activity and facility for long term use of the labeled product. The tissue receptors of individuals falling into the categories mentioned above will also be tested for their sensitivity to insulin.